This invention relates to voltammetric apparatus, more particularly improved apparatus for use in differential pulse anodic stripping voltammetry (hereinafter abbreviated as DPASV).
DPASV is recognized in the art as an effective method for analyzing a minute quantity of components, especially metals contained in solutions.
The method of DPASV consists essentially of the following two steps. In the first step a plating voltage about several hundreds millivolts negative with reference to the oxidation reduction potential of the component to be analyzed is impressed upon a working electrode immersed in a solution to be measured for plating the working electrode. In the second step a voltage comprising direct current voltage (stripping voltage) which increases gradually with time in the positive direction from the plating voltage and superimposed upon a pulse voltage is applied by sweeping upon the working electrode for stripping the component that has been plated on the working electrode during the first step. For this reason, the second step is termed the stripping step. FIG. 1 of the accompanying drawing is a graph showing the manner of applying the voltage in the first and second steps. Although in FIG. 1 the pulse voltage is shown as being superimposed upon the stripping voltage (sweep potential) on the positive side thereof, it is also possible to superimpose the pulse voltage on the negative side of the stripping voltage.
In sweeping during the second step, among the current flowing through the working electrode, the current that flows while the pulse is not applied, preferably the current that flows during a definite interval (sampling time S.sub.1) immediately prior to the application of the pulse voltage, and the current that flows while the pulse is applied, preferably a definite interval of the latter half of the pulse (sampling time S.sub.2) are measured to obtain sampling currents IS.sub.1 and IS.sub.2 respectively and the difference between these sampling currents is determined. FIG. 2 diagrammatically shows one example of the relationship between the pulse and the sampling times S.sub.1 and S.sub.2.
To calculate the difference between sampling currents IS.sub.1 and IS.sub.2, these currents are usually converted into corresponding sampling voltages which are held by respective voltage holders and then the difference between these sampling voltages is determined. The maximum value or the integrated value of the voltage difference is used to calculate the concentration of the component to be analyzed.
The outline and present state of DPASV can be found in the following papers.
A. J. B. Flato, Analytical Chemistry, Vol. 44, September 1972, pages 75A-87A. PA1 b. H. Siegerman et al., American Laboratory, Vol. 4, No. 6, pages 59 - 68 (1972) PA1 c. T. R. Copeland et al., Analytical Chemistry, Vol. 46, No. 14, Dec. 1974, pages 1257A - 1264A.
One example of the apparatus for use in DPASV is described in U.S. Pat. No. 3,420,764.
Although DPASV is an analytical method having an extremely high sensitivity, it is desirable to improve further the sensitivity.
As a result of an exhaustive investigation regarding the method of voltage sweeping of the working electrode we have found that the current flowing through the working electrode varies in a different manner according to the period (repetition period) of the pulse voltage. More particularly, where the pulse period is long, for example from 0.5 to 5 seconds, the sample current IS does not contain any peak as shown in FIG. 3 whereas sample current IS.sub.2 contains peaks corresponding to the components to be analyzed. As has been pointed out before where the pulse voltage is superimposed upon the stripping voltage on the negative side thereof, curve S.sub.2 will be positioned beneath curve S.sub.1 with its direction inverted. In the case shown in FIG. 3, the difference between IS.sub.2 and IS.sub.1, is the output signal and is shown by FIG. 4.
Where the period of the pulse is extremely short, for example 116.9 milliseconds, as shown in FIG. 5 although the peaks of sample current IS.sub.2 do not change sample current IS.sub.1 contains peaks of the opposite polarity and somewhat lag with respect to the peaks of the sample current IS.sub.2. Where the pulse voltage is superimposed on the negative side of the stripping voltage the polarities of curves S.sub.1 and S.sub.2 are reversed and curve S.sub.2 is positioned beneath curve S.sub.. In any case, the difference between sample currents IS.sub.2 and IS.sub.1, is the output signal and is shown by FIG. 6.
In this manner, the output signal increases as the pulse period decreases and the sensitivity is improved in proportion thereto.